ruby on rails - Find document in subarray in mongodb -

i have many collections such this

{ "_id" : 383566, "horses" : {     "609075" : {         "_id" : 609075,         "name" : "aghawonan"     },     "634478" : {         "_id" : 634478,         "name" : "french thyne"     },     "595535" : {         "_id" : 595535,         "name" : "doctor hanley"     },     "593718" : {         "_id" : 593718,         "name" : "ballyhill boy"     },     "578089" : {         "_id" : 578089,         "name" : "oh alabama"     },     "632597" : {         "_id" : 632597,         "name" : "bowling alone"     },     "525424" : {         "_id" : 525424,         "name" : "tally-ho duke"     } } 

i need find collections in array horses _id have match

my version, not work

db.cards.find({'horses': {$elemmatch: {_id:525424}}}) 

in example, horses not array, object. data structure bit odd , redundant because id both part of 'name' (or path if will) , actual document data. in data structure, refer particular horse horses.609075.name, not helpful.

your query work if horses array so:

{   "_id" : 383566,   "horses" : [     {         "_id" : 609075,         "name" : "aghawonan"     },     {         "_id" : 634478,         "name" : "french thyne"     },     // ...    ]  } 

since you're querying single property, $elemmatch isn't required. query this:

db.cards.find({"horses._id" : 609075}); 

please note you'll receive entire card result, not part of array.

as side note, why ids of horses called _id? name _id special field @ root level. embedded documents don't have special fields.